How to tween the alpha channel (visibility) of an object?
The outliner displays ALL object in the scene independent of keyframes. You can add and delete objects from your scene anytime on any keyframe.

However if you want your object to show up at a certain keyframe just animate its visibility by pressing I while hovering over the visibility icon of the object thus adding a keyframe for that property.

You could also do a search for the answers, since you are not the first one to have this kind of problem ;)
https://stackoverflow.com/questions/8844389/how-to-make-an-object-invisible-at-a-particular-keyframe-without-moving-it-in-bl